In this Section you will be studying two similar drawings: schematic
and logic diagrams. These drawings usually come after the preliminary
drawings, the block and single line diagram.

SCHEMATIC DIAGRAMS

The schematic drawing is a symbolic representation of the data and components
used in an electronic circuit. You will learn how the schematic uses
symbols from the electronic language. Service, sales, manufacturing,
and engineering people can not adequately communicate about an electronics
device without the help of a schematic or logic drawing. The way the
components are connected will inform the reader of their function. The
function will be further explained when you add values, ratings, tolerances,
and catalog numbers to the components, FIG. 1.

Engineers and drafting supervisors give you sketches of the schematic
from which to work. The sketches given to you as a beginning drafter
will be formally laid out. Then as your skills and knowledge increase,
the sketches will be only rough layouts of the circuit, FIG. 2. A schematic
drawing can be laid out in many ways. No two drafters will organize the
drawing exactly the same way. However, they should follow some basic
principles. These principles will be discussed later in this Section.

FIG. 1. A—Schematic of a transistor amplifier circuit. B—Add more information
chiefly to help a technician troubleshoot the circuit.

1. To organize the schematic to fit a specified paper format. Often
schematics must fit into books. This requires you to draft your drawings
to a size that will fit the book or can be reduced appropriately to fit
the book.

2. To apply proper symbols and reference data for each component. This
will be accomplished by having an understanding of American National
Standard Institute’s (ANSI) Y32. 1 6 Reference Designations for Electrical
and Electronics, Y32.2 Graphic Symbols for Electrical and Electronics
Diagrams, and Y32.14 Logic Diagrams, Graphic Symbols (AIEE/IEEE91).

3. To create general notes, legends, or detailed notes that explain
specifics of the schematic.

4. To follow the general rules when drafting the schematic.

RULES FOR DRAFTING SCHEMATICS

All drawings have general rules for the way they are created. The schematic
should be drawn using the following rules:

1. Normal signal flow should be from left to right, top to bottom. An
example would be a radio circuit, FIG. 3. The antenna (input) should
be on the upper left of the paper. The speaker (out put) should be on
the right side.

2. Lines should be spaced a minimum of 3/8 in. (10 mm) apart.

3. Lettering should be 5/32 in. (4 mm) high. This requirement is necessary
for microfilming and

photo-reductions of the drawings.

4. Lines between components should take the shortest path.

5. Connecting lines should have a minimum of crossovers and joggles,
FIG. 4 A and B.

6. Long parallel lines should be arranged in groups, preferably three
to a group, FIG. 4 B and C.

7. Avoid four-way tie points or four-way junctions, FIG. 5.

8. Power sources should go up and ground lines should go down, as shown
in FIG. 6.

9. All lines will run horizontally or vertically and connect in 90-degree
corners, FIG. 7. A flip-flop or crossover circuit is the only exception
to this rule.

FIG. 3. A schematic showing left to right signal flow. Signal travels
from the antenna to the speaker on the right.

FIG. 4. An example of how to handle jogged, crossover, and parallel
lines. A—The eye cannot easily follow a line. B—Regrouping and periodic
spacing helps. C—Periodic thick lines are one common method of improving
readability.

FIG. 5. Junctions and crossover methods used on schematics. A—An outdated
method. B—Method without dot makes only one junction at a given place.
C—Method using single junction with dot is most preferred. It helps trip
reader’s eye when it scans a line. D—Avoid four-way tie points in case
you forget the dot.

FIG. 6. A—An example of power sources up and ground lines down. Some
engineers prefer the heavy bus lines for the ground and power ties. B—When
many lines will be crossed, use a common ground symbol and power symbol.
Note: The + 5V of the bottom circuit does not go to the top of the paper.
It simply goes up. This saves crossing lines and makes the drawing easier
to read.

FIG. 7. A—An example of normal lines. All are drawn horizontally or
vertically with connections made at right angles. B—A flip-flop circuit
is an exception. It uses diagonal lines to show the crossover function.

REFERENCE DESIGNATIONS

Reference designations are combinations of letters and numbers. FIG.
8. They are used to identify components shown on the schematic. The reference
designation should be located as close to the graphic symbol as possible.

FIG. 8. A—An example of components and their reference designations.
B—How to number the components in the proper order.

Numbering of components should be in a sequential order starting from
upper left and proceeding left to right, top to bottom. When items are
eliminated because of drawing revisions, the remaining items are not
renumbered. But you create a table to show the missing or sometimes forgot
ten numbers, FIG. 9.

Placement of references will be dictated by each company’s drafting
standard. Most companies will follow the methods shown in FIG. 10. To
save room on a crowded schematic, some companies prefer the metho1. When
the schematic is crowded, and lines must be spaced a minimum distance
apart, method (B) is the preferred way of referencing the components.

FIG. 12. Series and parallel circuits. The potential will change at
each junction. Note the numbers on the examples. Each number represents
a new or different potential.

LOGIC DIAGRAMS

Logic drawings are diagrams representing the logical elements and their
interconnections. They will most often not show the component’s elements
or internal details. They will use symbols and supplementary data to
describe the function of each element. Symbols for logic diagrams are
covered in Graphic Symbols for Logic Diagrams (two-state devices) ANSI
Y32.14.

TYPES OF DIAGRAMS

There are two main types of logic diagrams, a basic and a detailed diagram,
FIG. 15. The basic diagram shows logical functions and their relation
ships without reference to physical relationships. It uses logic symbols
to show the main concept of the circuit. Detailed diagrams take the basic
information and add specifics or non-logic data. This data may include
pin numbers, test points, and other necessary physical elements.

LOGIC ELEMENTS

There are a few elements used in logic diagrams. They are: AND, NAND,
OR, NOR, and INVERTER GATES, plus Operational Amplifiers, Flip-Flops,
Schmitt triggers, Decoders, Counters, Shift registers, and Oscillators.
These are some of the most frequently used elements.

FIG. 13. Effect of series circuit and parallel circuit. A— Lamps in
series. B—String of lamps are all out when in series circuit. C—Lamps
in parallel. D—String of lamps are on except for burnt bulb in parallel
circuit.

FIG. 14. A parallel-series circuit shown before, A, and after the drafter
rearranged it. Example B lines the components up so the lettering can
be done on a common line, making the circuit easier to read.

AND gate—A gate circuit with more than one in put terminal, FIG. 16.
No output signal will be produced unless a pulse is applied to all inputs
simultaneously. In binary circuits all inputs must be “1”to get an output
“1 “. If any of the inputs are zero, the output will be zero.

INVERTER gate—A circuit that takes a positive signal input and puts
out a negative signal, or vice versa, FIG. 17. It has one input and one
output. It is often called a NOT circuit since it produces the reverse
of the input.

NAND gates—A combination of a NOT and AND function, FIG. 18. It has
two or more inputs and one output. The output is logic “0” if all the
inputs are “1 “. If any input goes to “0” the output goes to “1 “. With
logic of the opposite polarity, this type gate becomes a NOR gate.

OR gate—The OR circuit performs the function of producing an output
whenever any one (or more) of its inputs is energized, FIG. 19.

NOR gate—A combination of an OR and NOT function, FIG. 20. It will have
an output of “0” if any input is logic “1” and is logic “1” only if all
the inputs are logic “0”. With an opposite logic polarity, this gate
can become a NAND gate.

Operational Amplifier (Op Amp)—An amplifier that performs various mathematical
operations. They can be used to add, subtract, average, integrate, and
differentiate. It may have a single in put and output, FIG. 21.

FIG. 17. An INVERTER (NOT) symbol, its schematic, and truth table.

FIG. 18. A NAND gate, its schematic, and truth table.

FIG. 19. An OR gate with schematic and truth table.

FIG. 21. A two input Operational Amplifier (Op Amp) with external resistors
added to the symbol.

FIG. 20. A NOR symbol, its schematic, and truth table.

FIG. 22. A—Flip-flop symbol and a schematic of the flip-flop function.
B—Another type of flip-flop.

Flip-Flop—A flip-flop, FIG. 22, is a device which is stable in either
of two states. When triggered by an input or clock pulse, the flip-flop
moves from one stable state to the other. For example, if one of its
outputs, called Q, starts at 1, it will be at 0 after the input pulse.

The most common flip-flop is the JK type. Another type is the set-reset
(RS) flip-flop. Some types require both an input pulse and an “enable”
pulse.

Schmitt Trigger—A bistable pulse generator in which an output pulse
of constant level exists only as long as the input is constant, FIG.
23.

Fig. 23

Decoder—A device for translating a combination of signals into one signal.
It is often used to extract information from a complex signal or coded
signal, FIG. 24.

FIG. 24. A decoder with four inputs and ten outputs.

FIG. 25. A counter circuit symbol. The device can count up or down
Counter—A logic device that counts input pulses, FIG. 25. It may count
input pulses and then out put after a predetermined number has been received.
The counter may count up or down. Shift register—A circuit which can
shift information from one flip-flop in a chain to an adjacent flip-
flop when it receives a clock pulse, FIG. 26.

Oscillator—An electronic device that generates alternating current of
predetermined frequencies,

FIG. 26. A—Shift register symbol. The unit is used for data transfer.
B—Four flip-flops used as a four-stage shift register.

FIG. 27. The oscillator circuit is designed to take a voltage impulse
and produce a current that periodically reverses. To properly draw oscillators
and other elements correctly in the logic diagram, you must follow some
basic rules.

Rules for drawing logic diagrams

1. Draw each device so that the input is on the left or top of the element.

2. Outputs of logic elements should go to the right or down.

3. The basic rules for schematics apply also to logic diagrams.

4. The numbering of logic elements will be by physical positions in
the equipment. This rule will differ from the schematic which is numbered
left to right and top to bottom disregarding physical position.

REVIEW QUESTIONS

1. Long parallel lines should be put in groups of _____ (how many).

2. List the drafter’s responsibility when drawing a schematic.

3. Which way should the schematic’s signal flow?

4. Why should we avoid four-way tie points?

5. What are reference designations?

6. In what symbol can diagonal wiring lines be used?

a. AND

b. Op Amp.

c. NOR

d. Flip-flop.

7. What is a parallel circuit? Sketch one.

8. What is the standard source for symbols for logic diagrams?

9. What are the two main types of logic diagrams?

10. List the basic logic elements.

PROBLEMS

PROB. 1. Draw a formal schematic of the panel- mounted components for
the AM-FM radio shown in FIG. 28. Draw on “A” size vellum. Add reference
designations and make appropriate changes to the symbols.

PROB. 2. Prepare a schematic of the AM transmitter shown in FIG. 2
9. Sketch the circuit layout before going to the final drawing.

PROB. 3. Draw a schematic of the transistor amplifier shown in FIG.
30. Add correct reference designations for all components.